As new emerging materials, single-walled carbon nanotubes (SWCNTs) have recently attracted extensive research interest due to their specific electrical, optical and mechanical properties. For different applications, the raw SWCNT materials have to be purified and enriched, as they contain metallic (m) and semiconducting (sc) single-walled carbon nanotubes, amorphous carbon, catalyst and other impurities. For example, sc-SWCNTs can be used as the active channel materials in field effect transistors (FET) in logic circuit and other electrical devices. Among various purification methods, comparatively, polymer extraction (PE) approach is a low cost and scalable process, and the sc-SWCNTs materials from this process also show quite high purity level.
Recently, conjugated polymer extraction (CPE) processes have been developed to purify single walled carbon nanotube (SWCNT) raw materials. Compared with other surfactant-based methods, such as density gradient ultra-centrifugation (DGU), gel chromatography and biphasic separation, CPE is simple, scalable and cost effective, thus possessing properties that are highly desirable for industrial applications. More importantly, the dispersed product is obtained as an organic solvent-based dispersion with relatively high tube content (e.g. up to ˜20%-50%). This leads to additional benefits in the application of SWCNT materials in device fabrication and performance.
However, one of the problems associated with the CPE process lies in the difficulty to remove conjugated polymer that remains on the sc-SWCNTs after purification or device fabrication. In other methods, surfactants are used to disperse tubes in solution. While these small molecules have a weaker interaction with sc-SWCNTs and can be easily removed from the sc-SWCNT surface by a simple rinsing step, a large excess of surfactant is required, which is undesirable in many circumstances. One advantage of the CPE process is that relatively low weight ratio (e. g. polymer/tube weight ratio <2) of polymeric dispersant is needed to form a stable dispersion, especially when conjugated polymers are used and/or at high concentration, when compared to small molecule surfactants that are present at a weight percentage of 95% or more.
Conjugated polymers have much stronger adhesion interactions with sc-SWCNTs, and can helically wrapped around the sc-SWCNTs. Furthermore, in non-polar organic media, the π-π stacking interaction between the conjugated polymer and the sc-SWCNT surface can be stronger than that in polar solvents. Even after thorough solvent rinsing, however, the polymer content in the dispersion can still be over ˜50% by weight. This will reduce the sensitivity of a sensor using sc-SWCNTs in a chemiresistor or transistor
One way to solve this problem is to use polymers with special chemical moieties that are introduced into the CPE process. These polymers can be metal-coordination polymers based on the interaction between ligand and metal ions, or H-bonded supramolecular polymers. These linkages can be easily broken by acid treatment such that the polymers will degrade into small units. Some polymers may contain degradable units, such as disilane, photocleavable o-nitrobenzylether and imine bonds. Other polymers may contain special units, such as azobenzene or foldable oligomers, so the conformation of these polymers can be changed by external stimuli, such as thermal isomerization or by using different solvents.
Although the aforementioned degradable polymers can be used for sc-SWCNT purification and/or dispersion, there are still some major drawbacks. For example, the polymer can only be partially removed after degradation; most of the degradations are carried out in solution; after polymer degradation, the sc-SWCNT will form bundles in solution which cannot be easily used for device fabrication; and none of the above polymer degradation was demonstrated on device surfaces after fabrication with the expectation that the sc-SWCNT would be likely removed from the surface.
Thus, there is a need for polymers that form stable sc-SWCNT dispersions, which can be easily removed from the sc-SWCNTs either in solution or post-device fabrication without removal of the sc-SWCNTs from the surface of the device.
The following documents (all of which are hereby incorporated by reference) disclose other classes of degradable and/or removable polymers for use in CPE processes for purifying CNT:    Pochorovski I, et al. J. Am. Chem. Soc. 2015, 137, 4328-4331.    Toshimitsu F, et al. Nature Communications. 5:5041, 9 pages.    Umeyama T, et al. Chem. Commun., 2010, 46, 5969-5971.    Lei T, et al. J. Am. Chem. Soc. 2016, 138, 802-805.    Lemasson F, et al. Chem. Commun., 2011, 47, 7428-7430.    Wang H, et al. Nano Today, (2015) 10, 737-758
A few of the polymers disclosed in these documents comprise heterocyclic N-containing rings, but none disclose tetrazine-based polymers.
In addition, US 2008/287638 and US 2013/0253120 (both of which are incorporated by reference) disclose classes of conjugated polymers that may contain a tetrazine group that associate with carbon nanotubes.
In particular, US 2013/0253120 discloses polyolefins, which may be tetrazine functionalized polyolefins, for modifying nanoparticles (including CNTs). However, this document does not disclose the class of s-tetrazine polymers for use in CPE extractions, nor does it disclose the degradability of these polymers.
US 2008/287638 discloses a class of “sticky” supramolecular polymers comprising a conjugated or electroactive segment (e.g. fluorenyl) and a “sticky” segment that non-covalently binds with the sidewall of the CNT, the sticky segment possibly comprising a tetrazine. However, this document does not disclose the particular class of s-tetrazine polymers, let alone any particular polymers comprising a tetrazine. Nor does it disclose the use of these polymers for purifying sc-SWCNTs in a CPE process. Furthermore, there is no discussion of the degradability of the polymers.
It is also known that tetrazines react with CNTs to form covalent bonds thereby breaking C═C bonds in the CNT framework. For example, Broza G. Chemistry & Chemical Technology, Vol. 4, No. 1 (2010), 35-45, discloses that tetrazines are known to form covalent bonds thereby breaking C═C bonds in the CNT framework.
U.S. Pat. No. 8,673,183; Li Z, et al. J. Am. Chem. Soc. 2010, 132, 13160-13161; and Li Z, et al. Macromol. Chem. Phys. 2011, 212, 2260-2267 (all of which are incorporated by reference), all disclose the class of s-tetrazine polymers for use in electronic devices, but not for use in association with carbon nanotubes (CNTs) and especially not for use in a CPE process to purify CNTs.
One area of device fabrication utilizing nanotubes is chemical sensors. Ellis et al (ChemPlusChem, 2016, 81, 1248-1265) provide a review of synthetic methods used to functionalize CNT-based gas sensors, specifically those sensors that target biologically relevant breath markers.
In addition, J. F. Fennell, Jr. et al (Angew. Chem. Int. Ed. 2016, 55, 1266-1281) discuss the use of nanowires (NWs) in chemosensor development.
T. Zhang et al (Nanotechnology 19 (2008) 332001 (14 pp)) review progress on the development of different types of carbon nanotube (CNT)-based nanosensors, with focus placed on the means used to improve the sensing performance (sensitivity, selectivity and response time) through the rational functionalization of CNTs with different methods (covalent and non-covalent) and with different materials (polymers and metals).
M. Meyyappan (small, 2016, 12, No. 16, 2118-2129) reviews the use of carbon nanotubes in gas and vapor sensing.
Although it is broadly disclosed that conjugated polymers that may contain a tetrazine group can associate with carbon nanotubes, it is also known that tetrazines can react with CNT to form covalent bonds, and none of the prior art contains examples of polymers containing a tetrazine group where the polymer associates with CNT.
Therefore it would not necessarily be expected that tetrazine polymers would associate with rather than react with CNT. In addition, there is no indication in the art that tetrazine-containing polymers would be useful in a CPE process for purifying CNTs, let alone the specific class of polymers of s-tetrazine polymers.
It has now been found that s-tetrazine based polymers can be used for sc-SWCNT purification, dispersion and device fabrication. Since s-tetrazine units can be easily decomposed by photo irradiation or thermal treatment, both in solution or on the device surface, the small molecules formed by decomposition can be washed away in solution or evaporated by laser irradiation or under vacuum in the solid state. This enables higher sensitivity sensors for gases, such as ammonia or nitrogen dioxide (NO2).